Field of the Invention
The present invention concerns a method and magnetic resonance system for automated determination of an acquisition related to an examination region for acquisition of a magnetic resonance data set.
Description of the Prior Art
To acquire magnetic resonance data sets, energy is always injected (radiated) into a three-dimensional volume with a transmission coil. The volume can be significantly smaller in one direction (the direction of what is known as the slice gradient) than in the two other spatial directions. Such a situation is referred to as the excitation of a slice—this is primarily the case in two-dimensional imaging, as well as in spectroscopic imaging. In contrast to this, the term “excitation volume” is prevalent in three-dimensional imaging and spectroscopic experiments. As used herein, the designation “acquisition volume” means the volume excited by energy injection within the scope of the acquisition of a magnetic resonance data set (and in fact of both an image data set and a spectroscopy data set) and read out during the readout phase, this volume delivering a signal contribution.
If portions of the excited volume are not situated in the read-out volume (as is known in some methods of “Arterial Spin Labeling” and “Time of flight” flow imaging, for example), these parts are not included in the resulting image. These are volumes that are prepared for saturation of moving spins and are situated outside of the acquisition volume, although spins flow from these external volumes into the acquisition volume. The acquisition volume is also designated as a volume of interest or VOI.
The excitable or excited volume is relatively freely adjustable in the slice direction. The minimum height of the volume is predetermined by the excitation pulse that is used (namely, its shape) and the gradient strength in the spatial direction in which the gradient is applied during the radiation of the excitation pulse. Since the slice direction is freely selectable, different minimum slice thicknesses accordingly result in different spatial directions.
The examination subject, however, is to be imaged (or data acquired therefrom) in the entire excited region in both of the other spatial directions (called the read direction and phase direction). In imaging acquisitions, a phenomenon known as aliasing (folding) can arise, meaning that the truncated part of the examination subject on the other side of the image migrates into the image and this leads to superpositions with other segments of the examination subject that are imaged there.
In contrast to this, in spectroscopic experiments without volume selection the problem exists that a superposition of desired and unwanted signals always results upon acquisition of signals from all regions of the examination subject within the field of view or radiation pattern of the coil; for example, signal intensities from different body regions superimpose in the spectrum generated from the raw data.
In order to limit the acquired signal to specific regions, different types of “single voxel spectroscopy” have been developed. In such methods, tissue surrounding the acquisition volume is saturated with a preparation module within the pulse sequence, and signals are subsequently acquired from the acquisition volume and the surrounding volume. Due to the pre-saturation, the tissue outside of the acquisition volume no longer delivers any contribution, or delivers almost no contribution.
The term ‘saturation” is generally understood as meaning that a volume is excited by means of an RF pulse or excitation pulse so that the longitudinal magnetization flips by 90° out of the steady state. A spoiler gradient is subsequently switched (activated) that dephases this magnetization, so it is no longer detectable. The procedure to thus saturate the magnetization outside of the acquisition volume is categorized under the term “outer volume suppression” (OVS). The volume in which magnetization is saturated with an RF pulse is called a saturation band. Multiple saturation bands are required to saturate the tissue outside of an acquisition volume; normally eight to twenty saturation bands are used.
In addition to this, it is known to selectively excite the acquisition volume, for example by means of a sequence of one 90° pulse and two 180° pulses. The method is called PRESS. However, it is disadvantageous that essentially only cuboid volumes can be excited.
In the acquisition of a magnetic resonance data set with the use of OVS, the procedure is typically executed as follows. One or more overview images are acquired first. In these images, the acquisition volume is marked or segmented by a user evaluates the images manually. This volume is further simplified so that it has an evaluable number of sides or individual surfaces. After this, the saturation bands are automatically placed around the acquisition volume so that the signal contribution from outside the acquisition volume is minimal. The actual data acquisition then takes place. The acquisition volume thus is surrounded by the saturation bands, i.e., its boundaries are established or defined by these saturation bands.
For example, this method to acquire a magnetic resonance data set is known from Venugopal et al., “Automatic conformal prescription of very selective saturation bands for in vivo 1H-MRSI of the prostate”, NMR Biomed. 2012, 25: 643-653. In this method, a complete automation is still not achieved because the acquisition volume or, respectively, the examination subject using which the saturation bands are placed is to be segmented manually.